A typical gas turbine engine has an annular axially extending flow path for conducting working fluid through a compressor section, a combustion section, and a turbine section. The compressor and turbine sections each have a number of bladed rotor assemblies each including a plurality of blades circumferentially spaced about and secured to the radially outer periphery of a rotor disk.
In a conventional bladed rotor assembly, the rotor disk has a plurality of axial slots around its radially outer periphery. The blades comprise a root, a platform, and an airfoil. The platform has opposite facing surfaces. The root attaches to one of the surfaces, the airfoil attaches to the other. The slots and the roots have complementary shapes, typically either a dove tail or a fir tree. The root mates with the slot and the blade extends radially outward therefrom. This type of rotor assembly is relatively heavy, thereby necessitating that the rotor disk be sufficiently sturdy, and thus heavy, in order to accommodate the stresses resulting from the heavy blade.
Alternatively, the blades may be secured by bonding or welding, to the rotor disc to thereby form an integrally bladed rotor assembly (IBR). A major advantage of an integrally bladed rotor assembly is that there is often no need for an extended blade root or a blade platform. The airfoil may be secured directly to the radially outer periphery of the rotor disk. The absence of an extended root and a blade platform results in a blade that is lighter than a conventional blade. A lighter blade enables the use of a less rigid and lighter rotor disk, in which case the integrally bladed rotor assembly is overall much lighter than a conventional bladed rotor assembly.
A preferred method for bonding or welding the blade to the rotor disk is a linear friction welding process. In such a process, a surface on the blade is contacted (interfaced) to a surface on the disk. The interfacing surfaces typically have complementary geometries, i.e. similar lengths and similar widths. The two parts are rubbed together, in a reciprocating (back and forth), somewhat linear type oscillatory manner. The axis of the oscillation is typically roughly aligned with the longitudinal (lengthwise) axis of the interface, i.e., end to end. As the parts are rubbed, compressive force is applied to place the interface under high pressure. At the interface, frictional heat is generated and material from each part changes to a molten or preferably to a plastic state. Some of this material flows out from between the parts (flash flow), resulting in gradual decrease in the thickness, i.e. the dimension in the direction in which pressure is applied (the dimension perpendicular to the interface) of the parts. When the process is terminated, flash flow ceases, and at the interface, the remaining plastic state material of each part cools and changes back to solid state, forming bonds therein and bonding the two parts together.
Linear friction welding of the blades to the rotor disk has been accomplished using two different methods. The first method interfaces a surface on the base of the blade to a slightly elevated surface, i.e., stub, on the periphery of the rotor disc. The axis of oscillation is roughly aligned with the chord of the blade. This method has several drawbacks. Fabrication of a disk with stubs is difficult and costly because it requires complex machining, e.g., 5 axis milling. Furthermore, there is significant concern over the strength of the stub, i.e., its ability to withstand the linear friction welding process. In an original equipment manufacture situation, the manufacturer can provide oversize part geometries, i.e., excess material, to give the stub sufficient structure and rigidity to withstand the welding process. Complex (and thus somewhat costly) machining is then used to remove the excess material and thereby obtain a final shape. In a repair situation, however, the damaged portion is removed, leaving the stub, but the stub is already at its final shape. Without added measures the final shape stub may not have enough rigidity to withstand the forces needed for linear friction welding.
The second method, disclosed in U.S. Pat. No. 5,366,344 to Gillbanks et al., interfaces a blade having a root of generally wedge-like form with opposed converging surfaces, with an axial slot having opposed diverging surfaces, in the periphery of the disc,. This method too has disadvantages. It may be more costly and difficult to use because it requires four interface surfaces, two on the blade and two on the disk, rather than just two interface surfaces. Moreover, with this method, flash material, which can contain impurities that cause weld defects, will collect in the bottom of the slot. Although this flash could be removed, such an operation is expensive and it leaves a hole in the base of the slot. Further, the use of axial slots may prevent the use of this method for some compressor rotors, because the chord of the airfoils would interfere with linear oscillation of the blade relative to the disc. Furthermore, the interface surfaces are not perpendicular to the direction of the compressive force, thus, greater compression forces may be necessary to achieve adequate pressure at the interface for bonding.
G. B. Patent No. 1,053,420 (app. no. 32751 (11/8/64)) to Petrie et al. discloses an integrally bladed disk that includes a disk having axial recesses, of curvilinear profile, around its periphery, and blades having roots secured, by welding, in the recesses. However, Petrie et al. do not mention linear friction welding. Furthermore, blades having a geometry disclosed in Petrie et al. can not be oscillated within the axial recess without interfering with adjacent blades. Rather, Petrie et al. suggest that the roots may be welded in the recesses by electron beam welding. Electron beam welding is unlike linear friction welding, in that with electron beam welding, the structures being welded are stationary, rather than oscillating, relative to each other throughout the welding process. Moreover, the integrally bladed disks disclosed by Petrie et al. employ conventional blades having roots, platforms, and airfoils, and thus they do not offer the major advantage sought from an integrally bladed rotor assembly, low weight. Furthermore, the roots extend, and thereby position the platforms and the airfoils, radially outward from the periphery of the disk.